A standard home tumble laundry drier condenses a stream of hot air blown into a drying drum and which removes moisture from the laundry; and the front access to the drum is closed by a hinged panel-type front door. More specifically, a known laundry drier comprises a ventilation system (i. e. usually a blower comprising a fan and an electric fan motor) and a heating arrangement, which draw air from the outside and, via an appropriate conduit arrangement, heat and blow the air into and through the laundry drying drum. The hot drying air is then either exhausted directly from the drier or fed to condensing means to condense the moisture collected in the hot air.
In the past, the duration of a drying cycle was constant and predetermined. However, the weight and initial humidity of the laundry to be dried are variable, so that a drying cycle of fixed duration may be either too short (i.e. at the end of the drying cycle, the laundry is still too damp and the drying cycle therefore ineffective) or too long (i.e. the drying cycle has used too much energy and is therefore inefficient).
A modern tumble drier normally employs a sensor to measure the relative humidity of the laundry during the drying cycle and to stop the drying cycle when the humidity of the laundry reaches a given value depending on the drying cycle selected by the user. The most effective way of measuring humidity is a direct measurement of the conductivity of the laundry. Various solutions are already marketed, which measure the conductivity between the drum and metal inserts fixed to the drum outlet or the lifters, or in which the drum is divided into two halves, and conductivity measured in between.
A limitation of this last method lies in the restrictions imposed on the drum which, in this case, must be made of any conductive material (e.g., stainless steel) and cannot be lined with soft materials, such as thin silicone layers, because they are insulating. Consequently, the last method cannot be used in a tumble drier, in which “gentle treatment” of the laundry is achieved by lining the drum with soft materials.
To design the drum with no restrictions imposed by the humidity sensing system, a new so-called “Limited Conductimetric System” has been proposed, based on a pair of small electrodes fixed to a not-moving part of the machine, e.g. the inside of the door. The “Limited Conductimetric System” (today very common in the tumble dryers on the market) has a number of disadvantages: due to the limited contact surface between the electrodes and the laundry, this system is fairly unreliable in stopping the drying cycle in time, especially with small loads (for example, of less than 1 kg) and damp cycles (for example, a final humidity of over 3-4%). Even with standard loads and dry cycles, in some cases, problems may arise because the end-of-cycle condition is not totally repeatable. Tests show that a tumble drier using the “Limited Conductimetric System” rarely stops a drying cycle with less than a 1 kg load in time; and, for damp cycles, even a 2 kg load may be an issue.
In other words, with electrodes of this sort fixed to the inside of the door, it is fairly difficult (if not impossible) to design an algorithm reliable enough to stop the drying cycle in time in the case of small loads and/or damp cycles.
FIG. 1 shows a chart (relative to a roughly 3 kg load) comparing a first voltage signal measured using a “Traditional Conductimetric System”, in which the drum is divided into two halves and conductivity measured in between (dotted line), and a second voltage signal measured using the “Limited Conductimetric System”, in which a pair of small electrodes are fixed to the inside of the door (continuous line). In the “Traditional Conductimetric System” the laundry is always in good contact with the conductimetric system, even with very small loads (less than 1 kg), so the voltage signal is very smooth and regular (dotted line in FIG. 1). Conversely, in the “Limited Conductimetric System”, the contact surface between the two electrodes and the laundry is fairly limited, so the voltage signal is irregular (continuous line in FIG. 1). Furthermore, the FIG. 1 chart relates to a 3 kg load; if we consider smaller loads, the situation with the “Limited Conductimetric System” is even worse, whereas the traditional conductimetric system is always reliable.
U.S. Pat. No. 4,531,305 discloses a laundry drier, in which the electrical resistance of wet articles and exhaust air temperature are monitored. The instant the monitored electrical resistance reaches a predetermined value, the time-varying rate of change of the monitored temperature is detected to estimate how long the drier is to be kept running; and, at the end of the estimated time period, the heat cycle of the drier is shut down.
EP0388939 discloses a laundry drier comprising, in a casing, a rotary drum for wet laundry, a motor for driving the rotary drum, electric heaters for heating the laundry, a temperature sensor for detecting a temperature in the rotary drum, an absolute humidity sensor for detecting absolute humidity in the rotary drum, and a control device for controlling operation of the laundry drier in response to outputs of the temperature sensor and absolute humidity sensor. The control device comprises circuit devices for powering the electric heaters in response to the output of the absolute humidity sensor.
EP1420104 discloses a process for drying laundry in a laundry enclosure or drum of a drying device, such as a drying machine, washer-drier or drying cabinet, and comprising one or more steps of drying the laundry in an airflow heated by heating means and fed into the laundry enclosure by ventilation means, and a step of ventilating the laundry in an airflow at ambient temperature fed into the laundry enclosure by the ventilation means, and in which the drying process begins with the ventilation step, to reduce the initial peak moisture content of the laundry by means of the airflow at ambient temperature. One embodiment also uses sensor means to detect the moisture content of the laundry, for example, by measuring the electrical conductivity of the laundry. This is done using a conductivity sensor (not shown in the drawings) comprising at least two metal electrodes (in contact with the laundry) inside the laundry enclosure. The power supplies to the heating means and fan means, and hence the duration of the individual steps, are calculated by a control unit on the basis of the temperature and/or conductivity readings, and optionally of the values entered by the user, and taking into account the laundry moisture reduction target values, particularly during the ventilation step.